Study On Phosphorus Removal By Adsorption Of Activated Alumina Modified By Iron

Controlling phosphorus discharge is of great significance to prevent water eutrophication pollution.Activated alumina and iron oxide are excellent adsorbents for phosphorus removal by adsorption method.However,activated alumina has a low adsorption capacity for low-concentration phosphorus containing wastewater.In addition,because of the small particle size of iron oxide,when it is directly applied to the aqueous phase adsorption system,the loss is serious and the recovery is difficult.In view of the problems in the application of two kinds of adsorbents in phosphorus removal from wastewater,three kinds of iron salts,ferric chloride,ferric sulfate and ferric nitrate,and two methods of water bath heating and static precipitation are used to modify the surface of granular activated alumina.Six kinds of iron modified activated alumina composite adsorbents were prepared.One of the best adsorbents was selected and correlation characterization analysis was also carried out.The performance of phosphorus removal,adsorption mechanism,adsorbent regeneration and phosphorus recovery were further studied.The phosphorus removal effects of composite adsorbents prepared by different preparation methods and theoretical iron contents were compared.The optimized adsorbents Iron modified active oxidation aluminum（FAA）were prepared by water bath heating method and ferric chloride,and the theoretical mass percentage of iron was 10%.It can be seen from the characterization results that the actual iron content of FAA is 19.61mg/g,and the surface of FAA is coated with rough sheet-like loading layer.The loading layer is mainly composed of Fe OOH and Fe₂O₃.Due to the appearance of the loading layer and the change of pore structure,the average pore diameter and pore volume of FAA increased compared with the original activated alumina,but the specific surface area decreased slightly,which was not different from that of the activated alumina.In addition,the ability of the modified adsorbent to adsorb phosphate anions by electrostatic attraction was enhanced as the isoelectric point of the modified adsorbent enhanced.The research results of FAA adsorption phosphorus removal performance show that the phosphorus removal rate of FAA increases with the increase of dosage.FAA has an excellent phosphorus removal effect under neutral or slightly acidic conditions.In the range of pH 2-7,the removal rate is greater than 80%.FAA has excellent metal stability in the range of pH=4～10.The coexistence of Cl^-and NO₃^-has little effect on the phosphorus adsorption capacity of FAA,while the coexistence of SO₄^2-has a greater negative effect on it,but FAA still has the best selectivity to H₂PO₄^-among the anions studied.The Freundlich model and Tempkin model have a better fitting effect on FAA adsorption isotherms（R²=0.96～1.00）,so the adsorption reaction is dominated by multilayer adsorption,and at the same time there will be interactions between adsorbates,which will affect the adsorption process.The pseudo-second-order model and the intra-particle diffusion model can better describe the kinetic process of FAA adsorption.Therefore,the reaction rate of phosphorus removal by FAA adsorption is mainly affected by chemical adsorption and mass transfer process.As for the adsorption thermodynamics,FAA adsorption of phosphate is an exothermic reaction that proceeds spontaneously.FAA treats the actual secondary effluent,the total phosphorus of the effluent reaches the landscape reuse water requirement,and the removal rate of soluble reactive phosphorus reaches 96.87%.Through the relevant characterization analysis of the FAA before and after the adsorption of phosphorus,it was found that the ion exchange of chloride ion and phosphate ion,the coordination reaction of phosphate and metal hydroxyl to form bidentate mononuclear inner-sphere complex and surface chemical precipitation of FePO₄ formation are the main way for FAA to adsorb phosphate.The research results of regeneration and phosphorus recovery show that the1mol/L NaOH solution has the best single desorption effect on FAA after phosphorus adsorption.The desorption rate can reach 93.56%after 9h.After regeneration,the phosphorus removal rate of the adsorbent only decreased by0.77%compared with that of the initial adsorption.However,the 0.1mol/L NaOH solution is more suitable for multiple desorption regeneration.Although its adsorption effect after a single desorption regeneration is slightly worse than that of the high-concentration NaOH solution,it can enhance the sustainablility of FAA.Calcium phosphate precipitation method is used to recover phosphorus from0.1mol/L NaOH desorption solution.The phosphorus recovery rate increases as the molar ratio of calcium to phosphorus increases.When the molar ratio of calcium to phosphorus is 6.67,the phosphorus recovery rate reaches 96.65%.At this time,the main component of the precipitated product is hydroxyapatite,and it also contains calcium hydroxide produced by side reactions during the recovery process.In conclusion,Iron modified activated alumina is a phosphorus removal adsorbent with good phosphorus removal effect,good selectivity,easy separation capacity and repeatable regeneration capacity,which can effectively achieve deep phosphorus removal in secondary effluent.